1. Field of the Invention
The invention relates to a cellular digital radio transmission system employing combinations of different modes of multiplexing for transmission of messages between the base stations and the mobile stations in such systems.
2. Description of the Related Art
There are three known basic methods for sending digital messages over a transmission medium (e.g. transmission line, radio channel): these are code-division multiplexing, frequency-division multiplexing and time-division multiplexing.
In the code-division multiplexing method the different messages sent over a common transmission medium are imposed by, for example, baseband modulation on a carrier and the resultant narrow-band signal (narrow in relation to the bandwidth of the channel) is spectrally spread over the channel bandwidth by multiplex modulation using a codeword that characterises the receiver.
Recognition of the signal at the receiver takes place not by time-division or frequency-division selection but on the basis of the spectral coding. The multiply stacked spectrally-coded messages in the code-division multiplex channel are selected in the receiver on the basis of their allocated codeword.
In the frequency-division multiplexing method the whole bandwidth available for message transmission is divided into narrow frequency bands, each of which corresponds to one message transmission channel. This narrow frequency band is at the disposal of the user for the duration of the message transmission.
In the time-division multiplexing method each user has available to him the whole bandwidth of a single transmission channel but only for short intervals of time. The characters or sequences of characters of various users are interleaved and are sent out with a correspondingly higher bit rate, the time channel allocated to each user being periodically repeated after the duration of each frame period.
From West German Pat. No. DE-OS 25 37 683 a radio transmission system with base stations and mobile stations is known, in which signalling channel access is effected by either asynchronous time-division multiplex, code-division multiplex or frequency-division multiplex.
For codeword synchronization incoherent carrier demodulation is used. A code generator consecutively generates one of the nine different codes which characterize the base stations. After this code has been synchronized with a received signal the corresponding intermediate-frequency signal is multiplied by it, thereby transforming the broad received signal spectrum into the massage channel bandwidth. The received message can then be recovered by means of, for example, a DPSK demodulator. For synchronization, the message is prefixed by a characteristic code pattern with a length of, for example, 15 bits.
Also known are combinations of the aforesaid methods and their application in a digital radio transmission system. For example, in "Nachrichtentechnik, Elektronik+Telematic 38 (1984), Vol. 7, pages 264 to 268" a digital radio transmission system is described in which the time-division multiplex method is used in combination with spectral coding, but in which the various users are not separated by means of the code-division multiplex method. In the time channels for speech and/or data message transmission (TCH communication channel) a bit sequence for establishing synchronism, a frame-synchronization word and the bit sequence of the message itself are transmitted one after the other. The time channels for message transmission (3.times.20 TCH) are arranged with organization channels (3 CCH) to form a time-division multiplex frame with a duration of 31.5 msec. If the message to be transmitted is a speech signal, adaptive delta modulation can be used for analog/digital conversion. A code is superimposed on the resultant message characters (bits) in the sender. It has proved advantageous to group the individual message bits into blocks of four bits each and to spread the resultant blocks with an orthogonal alphabet. The spreading factor used as a compromise designed to combine the advantages of band spreading with the requirement to use the frequencies economically.
A message transmission method has also been proposed, see published West German patent application No. (P 34 47 107.3), corresponding to copending U.S. application Ser. No. 810,439 assigned to applicants' assignee, whereby a different modulation system is used for the forward and reverse directions of the message transmission channel. For message transmission to the base station the mobile stations use one of a multiplicity of communication channels. In the direction from the base station to its satellite mobile stations each communication channel is separated by bandspread modulation.
The spread communication channels are superimposed on each other and the resultant broadband sum signal is transmitted in a common frequency band. In the direction from the mobile stations to the base station the messages are transmitted in separate narrowband frequency channels.
For speech transmission from the base station to the mobile stations the bandspread modulation used is selected by the base station and communicated to the mobile station when building up the communication link. For the transmission of signalling to the mobile stations allotted to the base station, a common bandspread modulation is used for all mobile stations in the direction from the base station to the mobile stations.
To distinguish between base stations configured in neighbouring transmitting cells, these stations transmit to the mobile stations in different frequency bands. The base stations are provided with narrowband receivers which, during operation, can be switched to several different frequency channels. The number of switchable transmitting frequencies in the mobile stations is smaller than the number of switchable receiving frequencies in the base station. For example, in the base station it is possible to switchover to 1,000 frequencies, whereas in the mobile station it is possible to switchover to 40 frequencies.
In each base station the receiving frequencies used are organized in order to optimize the interference situation. In the event of reception disturbances the relevant connection from the mobile station to the base station is switched to another undisturbed frequency channel to which both the base station and the mobile station can changeover. The receiving connection in the base station to the public telephone system continues to take part in the connection.